Frequently metal work pieces, especially steel and other ferrous metal work pieces, which have surfaces that are to be subjected to wear by friction, abrasion, rolling loads, etc., are subjected to thermal treatments in which carbon and/or nitrogen is thermochemically diffused into the surface of the article to provide a case that is more abrasion and wear resistant than the underlying original metal. Such processes are called nitriding, carburizing, and carbonitriding. These processes take place at elevated temperatures varying from as low as about 950.degree. F. for nitriding to as high as 1700.degree. F. and higher for carburizing. The treatments may take place in gaseous atmospheres, fused salts, vacuum, fluidized beds, or in a granular packed medium. However the process is carried out, its function and purpose is to provide a thin case of chemically altered material on the surface of the work piece or article being treated that is harder and more wear resistant than the starting material, by introducing carbon and/or nitrogen into the surface layer which carbon and/or nitrogen reacts with some of the material in this outer layer forming a harder more abrasion and wear resistant microstructure.
It is often desired that only certain areas or portions of the surface of certain articles or work pieces be hardened, and that the remaining portions be retained with the original microstructure and composition of the article without the addition of diffused nitrogen and/or carbon. One technique for accomplishing this is to case harden the whole piece and then remove the hardened case material, as by grinding, where the hardened case is not desired. This has many drawbacks, and it is much preferred to selectively cover surfaces where case hardening is not desired, prior to the heat treatment of said work pieces. Workers in the art have long sought an effective, efficient way of accomplishing such selective addition of nitrogen and/or carbon. Such research has usually involved the application of some material to the surface of the work piece which will act as a barrier to the diffusion of nitrogen and/or carbon into the surface at those locations where the material is applied. This has resulted in several materials and techniques for selectively applying them, often referred to as "stop-off" or "masking" materials. The characteristics of such stop-off materials includes effectiveness for blocking nitrogen and/or carbon diffusion at operating temperatures, ease of application, ease of removability, does not introduce any adverse effects on the surface where applied, and preferrably is economical, non-toxic, readily available, and will produce uniform repeatable results under similar conditions.
There have been several prior art proposals for such stop-off techniques. One such technique that is frequently used is to plate a thin coat of metal, such as copper or nickel, onto the surface where diffusion is to be prevented. These metal masks can work well as a barrier against nitrogen and/or carbon diffusion, but it is difficult to selectively apply them and expensive equipement and techniques are needed to both apply and remove the coated metals. Other techniques use various organic and inorganic materials in a variety of binders to prevent nitrogen and/or carbon diffusion, but they are difficult to apply and have not proven wholly satisfactory in use, especially when used in fused salt baths.